746 



CHORDATA. 



sp.c 



but at the gastrula stage, and after the mesoblast has become 

 formed, the cells it gives rise to are enclosed in the epiblast, and 

 do not migrate inwards till a later stage. The products of the 

 division of the generative cell subsequently divide into two 

 masses. It is not possible to assign the generative cell of Moina 

 to a definite germinal layer. Grobben, however, thinks that it 

 originates from the division of a cell, the remainder of which 

 gives rise to the hypoblast. 



Chordata. In the Vertebrata, the primitive generative cells 

 (often known as primitive ova) are early distinguishable, being 

 imbedded amongst the cells of two linear streaks of peritoneal 

 epithelium, placed on the dorsal side of the body cavity, one on 

 each side of the mesentery (figs. 405 

 C and 4io,/0). They appear to be 

 derived from the epithelial cells 

 amongst which they lie ; and are 

 characterized by containing a large 

 granular nucleus, surrounded by a 

 considerable body of protoplasm. 

 The peritoneal epithelium in which 

 they are placed is known as the 

 germinal epithelium. 



It is at first impossible to distin- 

 guish the germinal cells which will 

 become ova from those which will 

 become spermatozoa. 



The former however remain with- 

 in the peritoneal epithelium (fig. 41 1), 

 and become converted into ova in a 

 manner more particularly described 

 in Vol. II. pp. 54 59. 



The history of the primitive 

 germinal cells in the male has not 

 been so adequately worked out as in 

 the female. 



The fullest history of them is 

 that given by Semper (No. 559) for 

 the Elasmobranchii, the general ac- 

 curacy of which I can fully support ; 



FIG. 410. SECTION THROUGH 

 THE TRUNK OF A SCYLLIUM 

 EMBRYO SLIGHTLY YOUNGER 



THAN 28 F. 



sp.c. spinal cord ; W. white 

 matter of spinal cord ; pr. poste- 

 rior nerve-roots ; ch. notochord ; 

 x. sub-notochordal rod ; ao. aorta ; 

 mp. muscle-plate ; mp'. inner layer 

 of- muscle-plate already converted 

 into muscles ; Vr. rudiment of 

 vertebral body ; st. segmental 

 tube; sd. segmental duct; sp.v. 

 spiral valve ; v. subintestinal vein ; 

 i>.o. primitive generative cells. 



